The communicating function of the PLC in the network system for a programmable controller in which the PLC having a communicating function and other communicating device (remote module) are connected to each other through the transmission path is divided into two types of communications such as communications with an input/output unit (described as a remote I/O module hereinafter) having only ON/OFF data and communications with a remote terminal unit (described as a remote terminal module hereinafter) having ON/OFF data and processing data.
Generally, in the former type of communications (remote I/O module), even the input and output units located at a remote site have a configuration in which data is fixed by each station and is communicated without determining a type of input/output unit (indicating an input unit, an output unit, and an I/O mixed unit) and a type of data units (indicating 16 data units, 32 data units, and 64 data units) so that it is not required to especially grasp these link data in the PLC side.
In the latter type of communications (remote terminal module), a remote station itself has various types of data including ON/OFF data, processing data, and message data, so that, in a case where efficiency of the network is considered, it is required to check a connecting state of the remote station in a master station (PLC).
Accordingly, in the network system for a programmable controller including therein a remote terminal module, link data such as a number of stations for connection and a number of input/output data is required to be set in the PLC.
Conventionally, for setting the link data, such measures as those described below are taken;
firstly, a switch for setting is provided in the master station; PA1 secondly, a specified area is provided in a device memory in the master station; PA1 thirdly, a transfer command for link data is programmed anew therein, and the link data is set according to a ladder program; and PA1 fourthly, specified parameters for the data are provided therein.
However, in the first setting system, a switch for setting has to be provided therein, so that upsizing and higher price of the device are inevitable, in the second and fourth setting systems, a particular area for linking is provided therein, so that increase of a memory capacity required is inevitable, and the particular area for linking is provided therein if the connection to a remote station is possible in a case where the area has not yet been connected thereto, so that an unnecessary processing time is disadvantageously added to the original processing time. Accordingly, a processing speed is reduced due to communications with a non-connected area.
The third setting system disclosed in Japanese Patent Laid-Open Publication No. SHO 61-137444 is required to execute a ladder program, so that increase of a step capacity required for the ladder program incorporated in the PLC can not be prevented, and if the ladder program is not set in the stand-by state for running even after a power is turned ON, the network is not established, and for this reason, data communications between communicating devices can not be established.